Secondary air supply for internal combustion engine

ABSTRACT

A secondary air intake system is used in an internal combustion engine. The system supplies secondary air through a first and a second secondary air passage. The first secondary air passage includes a first vacuum operated air control valve. The second secondary air passage includes a second vacuum operated air control valve and an electrically operated air control valve positioned in series. A vacuum control which includes an electrically operated vacuum control valve operates the first and second vacuum operated air control valves to vary the effective area of the two air passages. An air-fuel ratio control which includes an intake air temperature sensor, an exhaust gas oxygen concentration sensor and a throttle plate vacuum sensor determines the engine air-fuel ratio to intermittently open and close the second electrically operated air control valve to open and close the second secondary air passage. The air-fuel ratio control also operates the electrically operated vacuum control valve to gradually open and close the first and second vacuum operated air control valves to gradually open and close the first and second secondary air passages.

In internal combustion engines having a three-way catalytic converter inthe exhaust system to purify the exhaust gas, the three-way catalyticconverter is the most effective when the air-fuel ratio of the intakemixture is at its stoichiometric ratio of 14.7:1. The current practiceis to provide feedback control of the air-fuel ratio to the vicinity ofthe stoichiometric ratio in accordance with the concentration of theexhaust gas and the running state of the engine.

Typically the air-fuel ratio is controlled by providing a secondary airintake passage communicating with the downstream of the throttle valveof a carburetor. The air-fuel ratio is controlled by providing thesecondary air passage with an electromagnetic air control valve so thatthe valve may be opened or closed in response to the output signal of anoxygen concentration sensor positioned in the exhaust system. Whetherthe actual air-fuel ratio is lean or rich is determined from the outputsignal of the oxygen concentration sensor. The secondary air passage isopened to supply the secondary air to the downstream of the throttlevalve when it is determined that the air-fuel ratio is at a rich value.The secondary air passage is closed to interrupt the supply of thesecondary air when it is determined that the air-fuel ratio is at a leanvalue.

The air-fuel ratio is subject to a proportional (P) control by openingand closing the secondary air passage with the electromagnetic valve.The control rate for the air-fuel ratio is determined from the effectivearea of the secondary air passage. The difference in the flow rate ofthe secondary air between when the electro-magnetic valve is open andclosed is large because of the large effective area of the secondary airpassage. This makes it possible to control the air control valve over awide range. But when the effective area of the secondary air passage islarge the fluctuating range of the air-fuel ratio with respect to thedesired stoichiometric value is also large, being increased by thedifference in the flow rate of the secondary air between when theelectromagnetic valve is opened and closed. This results in a surgingrunning of the engine as exemplified by fluctuations in the enginer.p.m.

It is, therefore, an object of the present invention to provide asecondary air supply system to control the air-fuel ratio over the wideengine speed range while improving the running of the engine.

A secondary air supply system according to the present inventionincludes: means for determining an air-fuel ratio of the engine; firstand second secondary air passages communicating with the downstream ofthe throttle valve of a carburetor; a first vacuum operated air controlvalve positioned in the first secondary air passage for changing theeffective area of the first secondary air passage in accordance with thevacuum in a first vacuum receiving chamber; an electric-operated aircontrol valve positioned in the second secondary air passage inaccordance with the determined air-fuel ratio; a second vacuum operatedair control valve positioned in the second secondary air passage forchanging the effective area of the second secondary air passage inaccordance with the vacuum in a second pressure receiving chamber;vacuum control means for applying a control vacuum to the first andsecond vacuum receiving chambers so that the effective areas of both ofthe first and second secondary air passages may be gradually increasedor decreased to maintain a desired engine air fuel ratio.

The present invention will be described in connection with a preferredembodiment shown in the accompanying drawings, wherein:

FIG. 1 is a schematic diagram showing the preferred embodiment of thepresent invention;

FIG. 2 is a flow-chart illustrating the operation of the control circuitused in the embodiment of FIG. 1;

FIGS. 3a, b and c are waveform charts illustrating the secondary airflow of the embodiment of FIG. 1 under different control.

In the secondary air supply system according to one embodiment of thepresent invention shown in FIG. 1, primary intake air is supplied froman atmosphere suction port 1 through an air cleaner 2 and a carburetor 3to an engine 4. The carburetor 3 is provided with a throttle valve 5, aventuri 6 upstream of the throttle valve 5, and a choke valve 7 upstreamof the venturi 6. In the vicinity of the throttle valve 5 there isformed a throttle vacuum detection port 8 which is positioned upstreamof the throttle valve 5 when the throttle valve 5 is closed anddownstream of the throttle valve 5 when the throttle valve is open.

A secondary air passage 11 communicates between a point in the intakemanifold 9 downstream of the throttle valve 5 and a point in thevicinity of the air discharge port of the air filter 2. The secondaryair passage 11 is divided into two air control passages 11a and 11b toprovide control of the secondary air. The air control passage 11a isprovided with a vacuum operated first air control valve 12. This aircontrol valve 12 includes a vacuum chamber 12a, a valve chamber 12bforming a part of the air control passage 11a, a diaphragm 12c forming apart of the vacuum chamber 12a, a valve spring 12d positioned in thevacuum chamber 12a, and a tapered valve member 12e positioned in thevalve chamber 12b and biased through the diaphragm 12c by the valvespring 12d to close the air control passage 11a. The air control valve12 changes the effective area of the air control passage 11a inaccordance with the level of the vacuum prevailing in the vacuum chamber12a. The effective area of the control passage 11a is increased with theincrease in the vacuum and decreased with a decrease in the vacuum onthe vacuum chamber 12a.

The air control passage 11b is provided with a second air control valve13. This second air control valve 13 is constructed like the first aircontrol valve 12 such that it includes a vacuum chamber 13a, a valvechamber 13b, a diaphragm 13c, a valve spring 13d and a tapered valvemember 13e. The second air control valve 13 changes the effective areaof the air control passage 11b in accordance with the level of thevacuum prevailing in the vacuum chamber 13a such that the effective areais increased in accordance with the increase in the vacuum.

An air correcting passage 15 having an orifice 14 for idle correctionbypasses the second air control valve 13. In addition, the air controlpassage 11b is provided with an electromagnetic valve 16 downstream ofthe second air control valve 13. The electromagnetic valve 16 includes asolenoid 16a, a valve chamber 16b forming a part of the air controlpassage 11b, and a valve member 16c positioned in the valve chamber 16band operated by the solenoid 16a. The electromagnetic valve 16 closesthe air control passage 11b when the solenoid 16a is deenergized.

The first and second air control valves 12 and 13 have their vacuumchambers 12a and 13a communicating with the intake manifold 9 by way ofa vacuum supply passage 17. This vacuum supply passage 17 is providedwith an electromagnetic valve 18. The valve 18 includes a solenoid 18a,a valve chamber 18b forming a part of the vacuum supply passage 17, anda valve member 18c positioned in the valve chamber 18b and operated bythe solenoid 18a. The valve chamber 18b is vented to the atmosphere byway of an atmosphere supply passage 19 so that the vacuum supply passage17 is closed when the solenoid 18a is deenergized and so that the vacuumsupply passage 17 from the vacuum chamber 12a is made to communicatewith the atmosphere supply passage 19 through the valve chamber 18b.

The vacuum supply passage 17 connecting between the vacuum chamber 12aand the electromagnetic valve 18 includes a surge tank 20. The vacuumsupply passage 17 downstream of the throttle valve 5 from theelectromagnetic valve 18 includes a constant vacuum control valve 21, astorage tank 22 and a check valve 23.

The constant vacuum control valve 21 stabilizes the vacuum downstream ofthe throttle valve 5 to a predetermined level P_(r) when the vacuumexceeds this predetermined level. The check valve 23 only allowscommunication from a point downstream of the throttle valve 5 to theconstant vacuum control valve 21. The vacuum supply passages 17 at boththe sides of the electromagnetic valve 18 are provided with orifices 24and 25. The atmosphere supply passage 19 is provided with an orifice 26.

A control circuit 33 is connected to the solenoids 16a and 18a withdrive circuits 31 and 32. The control circuit 33 is connected to anoxygen concentration sensor 35 positioned in the exhaust manifold 34.The oxygen concentration sensor 35 generates an output voltage V₀₂ at alevel corresponding to the oxygen concentration in the exhaust gas. Theoutput voltage V₀₂ increases in accordance with the increase in theoxygen concentration.

An intake air temperature sensor 36 and a vacuum switch 37 are alsoconnected with the control circuit 33. The air temperature sensor 36 ispositioned adjacent the air cleaner 2. When the intake air temperatureexceeds a predetermined temperature t₁ the air temperature sensor 36generates a high level signal at a voltage V_(H). The vacuum switch 37also detects the load upon the engine. The vacuum switch 37 is turned onwhen the level of the vacuum in a pressure receiving chamber 37a islower than a predetermined level to generate a high-level signal at thevoltage V_(H). The pressure receiving chamber 37a communicates with thevacuum detection port 8 by a vacuum supply passage 38 which supplies avacuum P_(C).

The control circuit 33 shown in FIG. 2 includes a comparator 41 forcomparing the output voltage V₀₂ of the oxygen concentration sensor 35with a predetermined voltage V₁ corresponding to the stoichiometricair-fuel ratio; an inverter 42 connected with the output terminal of thevacuum switch 37; and an AND circuit 43 for taking the logic product ofthe comparator 41, the inverter 42 and the intake air temperature sensor36. The output signal from the AND circuit 43 is fed to the drivecircuits 31 and 32.

The intake secondary air supply system having the constructionpreviously described operates as follows. First in the operation of thecontrol circuit 33, when the ouput voltage V₀₂ of the oxygenconcentration sensor 35 is higher than the predetermined voltage V₁ (V₀₂is greater than or equal to V₁), the air-fuel ratio is in the rich rangeand the output level of the comparator 41 is a high level. When theoutput voltage V₀₂ is lower than the predetermined voltage V₁ (V₀₂ isless than V₁), the air-fuel ratio is in the lean range and the output ofthe comparator 41 is a low level.

This assumes that the air temperature sensor 36 and the vacuum switch 37inputs of the AND circuits 43 (other than the input coming from thecomparator 41) are at the high level. This occurs when (1) the intaketemperature is higher than the predetermined temperature level t₁ sothat the intake temperature sensor 36 is at the high level, and (2) thevacuum switch 37 is turned off because the level of the supply vacuumP_(C) to the vacuum switch 37 is higher than the predetermined pressureso that the inverter 42 also is at the high level. Therefore the outputlevel of the AND circuit becomes identical to the output level of thecomparator 41.

When it is determined from the output level of the oxygen sensor 35 thatthe air-fuel ratio is in the rich range, the output level of the ANDcircuit 43 is high and it is supplied as a rich signal to the drivecircuits 31 and 32. When it is determined that the air-fuel ratio is inthe lean range, the output level of the AND circuit 43 is low and it issupplied as a lean signal to the drive circuits 31 and 32.

The drive circuits 31 and 32 deenergize the solenoids 16a and 18a torender the electromagnetic valves 16 and 18 inoperative in response tothe lean signal and energize the solenoids 16a and 18a to render theelectromagnetic valves 16 and 18 operative in response to the richsignal.

When the electromagnetic valves 16 and 18 are inoperative theelectromagnetic valve 16 closes the air control passage 11b and theelectromagnetic valve 18 closes the vacuum supply passage 17. Theelectromagnetic valve 18 on closing establishes communication betweenthe vacuum chamber 12a and the atmosphere supply passage 19 to graduallyreduce the vacuum in the vacuum chamber 12a. As the vacuum in the vacuumchamber 12a is reduced the first air control valve 12 also closes theair control passage 11a. Since no secondary air is supplied from thesecondary air passage 11 to the engine 4 when the two air controlpassages 11a and 11b are closed the air-fuel ratio is controlled to therich range.

Next, when the electromagnetic valves 16 and 18 are changed from theirinoperative states to their operative states, the electromagnetic valve16 is instantly opened, and the electromagnetic valve 18 restores thecommunications of the vacuum supply passage 17 but interrupts thecommunication of the atmosphere supply passage 19 so that the vacuumP_(r) is supplied to the vacuum chambers 12a and 13a. As a result, thepressure in the vacuum chambers 12a and 13a gradually approach thevacuum P_(r) so that the openings of the first and second air controlvalves 12 and 13 are gradually opened to gradually increase therespective flow rates of the secondary air through the intake secondaryair passage 11 (air control passages 11a and 11b). When the pressure inthe vacuum chambers 12a and 13a, immediately before the supply of thevacuum P_(r), is substantially equal to the atmospheric pressure thefirst and second air control valves 12 and 13 are opened by the supplyof the vacuum P_(r) so that the secondary air begins to gradually flowinto the intake secondary air passage 11 until its flow rate is amaximum.

Next, when the electromagnetic valves 16 and 18 are changed from theiroperative states to their inoperative states, the electromagnetic valve16 is closed to instantly close the air control passage 11b. Theelectromagnetic valve 18 similarly closes the vacuum supply passage 17.The electromagnetic valve 18 also establishes communication between thevacuum chambers 12a and 13a and the atmosphere supply passage 19.Therefore, the vacuum chambers 12a and 13a are connected to theatmosphere so that their pressure gradually approach the atmosphere.Consequently, the first and second air control valves 12 and 13 aregradually closed so that the effective area of the air control passage11a is gradually decreased to reduce the flow rate of the secondary air.As a result, even with the air control passage 11b closed by the closingof the electromagnetic valve 16, secondary air continues to flow throughthe secondary air passage 11 from the air control passage 11a, since itsflow rate is gradually reduced with time.

When the air-fuel ratio is feedback-controlled to its stoichiometricvalue, the rich and lean signals are generated alternately butcontinuously. In the air control passage 11a, the secondary air flowrate is increased in the presence of the rich signal and is decreased inthe presence of the lean signal (shown in FIG. 3(a)) so that an integral(I) control is operated. In the air control passage 11b, the secondaryair intermittently flows (shown in FIG. 3(b)) and its flow rate isgradually changed so that an intermittent proportional integral (PI)control is operated. This results in the flow rate of the secondary airthrough the intake secondary air passage 11 to the engine 4 being thesummation of the secondary air flows through the air control passages11a and 11b (shown in FIG. 3(c)). In addition, the changes in thesecondary air flow rates vary because the respective time periods forthe lean signal and the rich signal vary depending on the running stateof the engine.

In a cruising state, the lean signal and the rich signal are generatedalternately but promptly so that the flow rate of the secondary airthrough the air control passage 11a becomes substantially constant. Thisresults in the air-fuel ratio being controlled to the vicinity of thestoichiometric value mainly by the flow rate of the secondary air whichintermittently flows through the air control passage 11b.

In an accelerating state, the air-fuel ratio is controlled to thestoichiometric value mainly by the increasing flow rate of the secondaryair through the air control passage 11a.

Next, if the air intake temperature is below the predeterminedtemperature t₁, the output level of the intake temperature sensor 36becomes low so that the output level of the AND circuit 43 becomes lowno matter how the output level of the oxygen concentration sensor 35might change. For a light-load running engine at a slow or idle speed,the throttle valve 5 is substantially closed so that the level of thevacuum P_(C) becomes lower than the predetermined level. This results inthe vacuum switch 37 being turned on to drop the output of the inverter42 to the low level so that the output of the AND circuit 43 takes thelow level. Therefore, even if the air-fuel ratio is in the rich range,the lean signal is fed to the drive circuits 31 and 32 so that thecontrol state of the air-fuel ratio is shifted from the feedback controlto an open-loop control. In this open-loop control, both theelectromagnetic valves 16 and 18 are inoperative to close the intakesecondary air passage 11 so that the air-fuel ratio is controlled to therich range from the stoichiometric value.

In the present invention two secondary air passages communicating withthe downstream of the throttle valve provide control of the secondaryintake air. The secondary air through one of the secondary air passagesis gradually increased or decreased by the integral control inaccordance with the air-fuel ratio signal. Also in accordance with theair-fuel ratio signal, the secondary air through the other intakesecondary air passage is caused to intermittently flow by the action ofthe first electromagnetic air control valve. This intermittentlysubjects the secondary air flow rate to the PI control. This permitsincreasing the integrally controlled flow rate of the secondary airthrough one of the secondary air passages and decreasing theintermittently controlled flow rate of the secondary air flowing throughthe other secondary air passage. This results in a reduction in thefluctuating range of the air-fuel ratio by the first electromagnetic aircontrol valve while permitting control of the air-fuel ratio over a widerange. This reduces the engine surging phenomenon, and improves controlof the exhaust emission resulting in improved running of the engine. Inaddition, the simplified construction achieves a lower manufacturingcost.

In a internal combustion engine provided with the secondary air supplysystem of the present invention, which may be used on a motorcycle orother vehicle, the vacuum downstream of the throttle valve can be highand constant at the start of the vehicle. This permits applying the highconstant vacuum to the vacuum chambers of the first and second aircontrol valves as soon as the rich signal is generated. This results inan immediate control of the air-fuel ratio to the lean range. Theshortened time before the air-fuel ratio takes the stoichiometric valuereduces the content of the hydrocarbons and carbon monoxide in theexhaust gas at the start of the vehicle.

The invention claimed is:
 1. A secondary air intake system for an internal combustion engine having a primary air intake passage with a throttle valve and an exhaust system comprising: means for determining an air-fuel ratio, first and second secondary air passages communicating with the downstream of the throttle valve of a carburetor, a first vacuum operated air control valve positioned in said first secondary air passage for changing the effective area of said first secondary air passage in accordance with the vacuum in a first vacuum receiving chamber, an electrically operated air control valve positioned in said second secondary air passage for opening and closing said second secondary air passage in accordance with said determined air-fuel ratio, a second vacuum operated air control valve positioned in said second secondary air passage for changing the effective area of said second secondary air passage in accordance with the vacuum in a second vacuum receiving chamber; and a vacuum control means for applying the same control vacuum to said first and second vacuum receiving chambers so that the effective areas of both first and second secondary air passages may be gradually increased or decreased to maintain a desired engine air-fuel ratio.
 2. The secondary air intake system defined in claim 1 wherein said vacuum control means comprise an electrically operated vacuum control valve which is operated by said air-fuel ratio determining means to gradually change the vacuum in said first and said second vacuum receiving chamber to gradually change the effective area of said first and second secondary air passages.
 3. The secondary air intake system defined in claim 1 wherein said air-fuel ratio determining means includes inlet air temperature sensing means, exhaust gas oxygen concentration sensing means and throttle valve vacuum sensing means for determining said engine air-fuel ratio.
 4. A secondary air intake system for an internal combustion engine comprising, a pair of parallel air passage means, a first air control means and a second air control means each positioned in a separate one of said pair of parallel secondary air passage means for parallel control of secondary intake air, said first air control means including vacuum control means for varying the volume of air flow in both of said parallel secondary air passage means and said second air control means including air-fuel ratio control means for gradually stopping and starting of air flow in both of said parallel secondary air passage means and intermittent stopping and starting of air flow in one of said parallel secondary air passage means.
 5. A secondary air intake system for an internal combustion engine having a primary air intake with a throttle valve intake manifold and an exhaust system comprising, a first and second secondary air passage supplying secondary air to the intake manifold, a first vacuum operated air control valve positioned in said first secondary air passage, a second vacuum operated air control valve and a electrically operated air control valve positioned in series in said second secondary air passage, a vacuum control means for simultaneously operating said first and second vacuum operated control valves to vary the air flow through said first and second secondary air passages, an air-fuel ratio control means for determining the air-fuel ratio during engine operation, said air-fuel ratio control means operating said electrically operated air control valve to intermittently open and close said second secondary air passage and operating said vacuum control means to open and close said first and second vacuum operated air valves to gradually open and close said first and second secondary air passages.
 6. The secondary air intake system defined in claim 5 wherein said air-fuel ratio control means includes an intake air temperature sensing means, exhaust gas oxygen concentration sensing means and throttle valve vacuum sensing means for determining said engine air-fuel ratio.
 7. The secondary air intake system defined in claim 5 wherein said vacuum control means includes an electrically operated vacuum control valve means for gradually opening and closing said first and second vacuum operated air control valves. 